The purity of the steam in the operation a power plant has to satisfy stringent requirements. In particular, it is important to avoid the situation where particulate solids are entrained in the steam. Such solid particles may lead to damage to the plant parts, such as, for example, the turbine. Damage in this case occurs particularly on the blading and the labyrinth seals of the turbine.
To protect the turbine and other plant parts against relatively large solid particles, steam screens are installed upstream of quick-closing interception valves or between quick-closing interception valves and the regulating valves in the steam lines. By contrast, solid particles of smaller diameter pass through the screens. They can therefore enter the turbine. These cause what is known as solid state erosion of the blading. This leads to material stripping and blade toughening and consequently to the impairment of turbine efficiency. In so far as the solid particles are braked by the turbine and they remain in the turbine, there is the risk that they lead to roughenings on the blading and possibly form deposits in the labyrinth seals.
These deposits may damage or destroy sealing elements or other components.
In boilers, steam lines or other steam-carrying components which lie upstream of the turbine in the steam flow direction, solid particles arise, in particular during the new installation of power stations. Solid particles occur even during plant inspections or component exchange. These solid particles are, in particular, rolling skin, scale, iron oxides and corrosion products and oxidation layers caused by the heat treatment of the plant components. Even with the greatest care during installation work, the situation cannot be certainly be avoided where dust, sand, installation objects and installation waste are left behind in steam-carrying components or plant parts of the plant. The impurities are sometimes present loosely. Sometimes, they adhere to the inner walls of the plant parts.
In general, therefore, as steam-carrying components, the boiler region, steam lines between the boiler and steam turbine or between the boiler and condenser, and also valves, have to be blown out or flushed out before every first steam pulse of a steam turbine and thereby freed of particles, in particular of solid particles, for example rolling skin, scale or iron oxides.
A scavenging method is employed for cleaning the power plant. After the scavenging or pickling of the plant has taken place, the steam-carrying systems are cleaned by means of steam. This is generated by the evaporation of water of highest purity (demineralized water) in the boiler. In this case, the water of highest purity is supplied to the power plant, led through the corresponding components and extracted again. Thus, for example, before every first steam pulse of a steam turbine, the steam-carrying boiler regions and the steamlines are freed of particles which may lead to damage and/or to an impairment of efficiency. This cleaning process is subject to various parameters, such as, for example, demineralized water generation capacity, demineralized water storage capacity and rapid demineralized water afterfeed, sound protection measures, resulting operational restrictions, evidence of freedom of the steam from particles, removal of temporary facilities, subsequent dependent commissioning steps.
In the cleaning of the current parts, temporary cleaning facilities connectable to the power plant are provided, which supply the scavenging medium to the power plant and extract again at least part of the scavenging medium after it has flowed through the components.
In conventional methods, the cleaning of the steam-carrying plant parts takes place by means of chemical cleaning (such as, for example, pickling) or by burning out with steam or by a combination of the methods.
In pickling, the cleaning waters are laden with a pickling agent, for example inhibited acid, complex formers, for example ethylene-diamine-tetraacetic acid, and therefore cannot be discharged into the environment. Considerable outlays are required for the disposal of the waste water laden with chemicals.
Superheaters, reheaters and associated steamlines are usually blown out with steam. In this case, it is appropriate to adhere to conditions which ensure that solid particles which have remained in the plant parts despite the blowing out with steam are not entrained by the operational steam flow during the operation of the plant.
Blowing out is therefore effective only at higher flow velocities than are to be expected when the plant is operating under full loading. According to experience, the dynamic pressure in blowing out amounts to 1.2 to 1.7 times the value of the dynamic pressure during operation at the maximum continuous output of the plant.
This blow-out condition can be fulfilled both in the case of continuous blowing out at relatively low pressure and in the case of discontinuous blowing out at relatively high pressure.
Both methods—pulse-like or continuous steam blow-out—are based on the fact that the steam discharged into the atmosphere has to be replaced by the afterfeed of fully deionized water (=dimineralized water). The steam cleaning duration is therefore always dependent on the demineralized water generation capacity or demineralized water storage capacity. Furthermore, complicated blow-out facilities are necessary in order to ensure a rapid afterfeed of demineralized water into the condensate system and in order to limit the sound emissions occurring during atmospheric steam blow-out to the required sound levels. For example, for this purpose, silencers are used, or water injection into the steam takes place and therefore leads to an additional water demand. Also, the conventional circulatory cleanings cannot be carried out uninterruptedly, since the assessment of the freedom of the steam from particles is not possible online in high temperature and pressure ranges.